1. Field of the Invention
The present invention relates to an apparatus for driving the front and rear road wheels of a motor vehicle, and moro particularly to an apparatus for driving the front and rear road wheels of a motor vehicle, which apparatus includes torque transmission clutches capable of adjusting the amounts of torque to be transmitted to a pair of laterally spaced auxiliary drive road wheels, and a speed increasing device disposed in a power transmission path connected to the driven wheels, the speed increasing device being capable of changing the speed of rotation.
2. Description of the Relevant Art
As shown in FIG. 1 of the accompanying drawings, there is known a front and rear road wheel drive motor vehicle (hereinafter referred to as a "4WD motor vehicle) in which drive power from an engine 1 is transmitted through a transmission T/M and an output shaft 2 thereof to a front wheel differential 3 comprising a differential case 4 and a pair of output shafts 5, 6 coupled to respective front road wheels FW through drive axles 7, 8, respectively, and then the drive power is transmitted from the front wheel differential 3 through a propeller shaft 9 to a rear wheel differential 13 for driving a pair of rear road wheels RW. The rear wheel differential 13 has a differential case 14 to which the rear end of the propeller shaft 9 is coupled through meshing gears 11, 12. The differential case 14 is combined with a pair of torque transmitting clutches (e.g., hydraulic multiplate clutches) 21, 25 mounted respectively on output shafts 15, 16 coupled to the respective rear road wheels RW through drive axles 17, 18, respectively. The hydraulic multiplate clutches 21, 25 are controlled by a hydraulic pressure control system (not shown) for varying the drive power to be transmitted to the output axles 15, 16.
More specifically, the hydraulic multiplate clutches 21, 25 comprise outer plates 22, 26, respectively, fixed to lefthand and righthand portions of the differential case 14 and inner plates 23, 27, respectively, fixed to the respective output axles 15, 16 and alternating with the outer plates 22, 26. The drive power to be transmitted to the output axles 15, 16 can be varied by introducing oil under pressure into hydraulic pressure chambers 24, 28 of the respective clutches 21, 25.
Now, it is assumed that the 4WD motor vehicle makes a turn as shown in FIG. 2. When the motor vehicle is smoothly steered while the engine power is small and the front road wheels FW are subjected to a small degree of slippage, the outer rear wheel RW (with respect to the turning circle) which is an auxiliary drive road wheel runs along a path r.sub.4 that is positioned outwardly of an average path f.sub.0 of the front road wheels FW which are main drive road wheels. Ideally, therefore, the rotational speeds .omega..sub.1, .omega..sub.2 of the inner and outer front road wheels FW, the rotational speed .omega..sub.0 of the propeller shaft 9, and the rotational speeds .omega..sub.8, .omega..sub.4 of the inner and outer rear road wheels RW should preferably meet the following relationship: ##EQU1##
In the 4WD motor vehicle shown in FIG. 1, however, even if the pressure applied to the hydraulic multiplate clutch 25 associated with the outer rear road wheel RW is increased, the rotational speed .omega..sub.4 does not exceed the rotational speed .omega..sub.0, but is equal to the rotational speed .omega..sub.0 at most. It has been unable to produce drive power to rotate the outer rear road wheel RW so that the relationship .omega..sub.0 &lt;.omega..sub.4 will be achieved. Consequently, the motor vehicle undergoes tight-corner braking when making a turn.
If the pressure applied to the hydraulic multiplate clutch 25 coupled to the outer rear road wheel RW were lowered, then the condition .omega..sub.0 &lt;.omega..sub.4 would be reached, but this would fail to accomplish the advantages of the 4WD motor vehicle resulting from the positive driving of all the front and rear road wheels.
The 4WD motor vehicle shown in FIG. 1 is structurally based on a front-engine, front-wheel-drive (FF) motor vehicle. However, the above problems are also associated with a 4WD motor vehicle which is structurally based on a rear-engine, rear-wheel-drive (RR) motor vehicle.
More specifically, FIG. 3 shows a 4WD motor vehicle in which engine power is transmitted from a transmission output shaft 2 to a rear wheel differential 3 and then transmitted from the rear wheel differential 3 through a propeller shaft 9 to a front wheel differential 13. The front wheel differential 13 has hydraulic multiplate clutches 21, 25, identical to those shown in FIG. 1, disposed in a differential case 14 and mounted respectively on front wheel output shafts 15, 16.
When the 4WD motor vehicle makes a turn as shown in FIG. 4, since the outer front wheel (with respect to the turning circle) which is an auxiliary drive road wheel runs along a path f.sub.2 that is positioned outwardly of an average path r.sub.0 of the rear road wheels which are main drive road wheels, the following relationship should preferably be met: ##EQU2## However, even if the pressure applied to the hydraulic multiplate clutch 25 associated with the outer front road wheel is increased, it is unable to produce drive power to rotate the outer front road wheel so that the relationship .omega..sub.0 &lt;.omega..sub.2 will be achieved.
This also holds true for a 4WD motor vehicle that is structurally based on a front-engine, rear-wheel-drive (FR) motor vehicle, not shown, and also for a midship-engine, rear-wheel-drive motor vehicle, not shown.
One solution to the above problem is to provide a speed increasing device having a speed changing capability and disposed in a drive power transmitting path from a differential coupled to main drive road wheels to a differential coupled to the auxiliary drive road wheels, the speed increasing device being controllable to make drive power applied to the main drive road wheels smaller than drive power imposed on the auxiliary drive road wheels. A drive torque applied to an outer one of the auxiliary drive road wheels can be larger than a drive torque applied to an inner one of the auxiliary drive road wheels by controlling a pair of torque transmitting clutches, thus improving operating performance or abilities, such as turning performance or ability, of the motor vehicle.
When the motor vehicle makes a turn, the speed increasing device in the power transmission path has to be switched to a speed increasing mode. If the speed increasing device is switched after the turn of the motor vehicle has been detected, however, a control time lag or delay is developed, and hence the response of the speed increasing device is limited.